The present invention relates to a method and arrangement for automatic gear-changing of a motor vehicle epicyclic change-speed gearbox from a previous gear, in which a first frictional connection is engaged by an associated pressure-medium gear-change actuator, into a new gear, in which a second frictional connection is engaged by an associated pressure-medium gear-change actuator and the first frictional connection is disengaged by switching off working pressure of the associated gear-change actuator, comprising means for measuring working pressure of a gear-change actuator of the new gear. The working pressure of the gear-change actuator of the previous gear is switched off when the working pressure of the gear-change actuator of the new gear reaches or exceeds a significant pressure value determined by comparison with a reference parameter.
In a known automatic gear-changing arrangement as seen in, for example, DE 38 41 304 A1, the working pressure of the gear-change actuator of third gear is fed, during a change from the second gear to the third gear, by an overlap control valve to a control pressure surface of a gear-change spool which controls the admission of working pressure to the gear-change actuator of the second gear. For this purpose, the gear-change spool and a second control pressure surface which, however, acts in the opposite direction and to which control pressure is admitted by an electromagnetic control valve when the gear-change spool is changed into its position for the second gear.
During this known procedure, the significant pressure value of the working pressure of the gear-change actuator of third gear is determined by the constant value of the control pressure and by the area ratio of the control pressure surfaces on the gear-change spool of the third gear. When the gear is changed back from third gear to second gear, another associated overlap control valve is actuated by feeding the working pressure of the gear-change actuator of the second gear to a control pressure surface of the overlap valve and by the resulting control pressure force being balanced by a spring force. In this arrangement, therefore, the significant pressure value of the working pressure of the gear-change actuator of the second gear, which leads to the switching off of the working pressure of the gear-change actuator of the third gear, is determined by a valve spring, i.e. again by a fixed reference parameter in the form of a design constant, as in the case of changing up.
A hydraulic pressure control system for an automatic gearbox with a torque converter is described in DE 39 36 115 A1. This known system operates with a multi-stage transmission gear with a plurality of gear wheel stages and friction elements for selecting power transmission paths within the gearbox, a hydraulic control device for controlling the engagement and separation of the friction elements in order to produce one of the gear stages, control apparatus for controlling the conduit pressure of the hydraulic control device as a function of vehicle operating conditions, an engine torque monitoring device for determining an engine driving torque, a monitoring device for determining a turbine rotational speed of the torque converter, and a gear-change condition monitor for determining an upward or downward change in the gearbox.
In order effectively to suppress a torque jolt during a gear change and to create a control of a conduit pressure as a function of an engine operating condition when changing the gear up or down, this known control system operates such that the conduit pressure control apparatus controls the conduit pressure as a function of at least the engine torque, when changing up, and of at least the turbine speed, when changing down. A hydraulic system pressure controlled by various parameters is regulated by a pulse valve when changing up. The pressure level, in addition to further devices, is intended to ensure that during gear changes, while the frictional element to be switched off has still not become unpressurized, the stress in the gearbox is kept within low limits. The actual time of changing up and down of the previous and new gear is exclusively determined by the time when magnetic valves, which control the switching valves setting the system pressure for the gear-change actuators of the frictional device, are actuated without the attainment of a reference pressure value being used as the switch-off criterion for the previous gear.
A method for gear-change control in an automatic gearbox which is drive connected to a drive machine with a load control element is also described in DE 41 14 383 A1. The automatic gearbox contains a predetermined rotational element, a disengaging element and an engaging element. So that a time at which an element being disengaged has to be released can be determined easily and rapidly, this known method includes the steps of initiating an engagement procedure of the engaging elements; recording a rotational speed of the predetermined rotational element and generating a rotational speed display signal which displays the recorded rotational speed; determining a differential coefficient with respect to time of the rotational speed display signal and generating a derived display signal which provides the differential coefficient determined; and releasing the disengaging element in reaction to the derived signal.
With this last-described known method, an attempt is made to determine the optimum switch-off time from the differential coefficient of the gearbox input rotational speed or angular speed with respect to time, with the derivative being calculated at equidistant time intervals. It is apparent, however, that changes to the angular speed can only occur when the engaging frictional element has already led to a retardation of the rotational speed and therefore to the travelling speed and, therefore, this signal always comes too late.
In contrast thereto, an object on which the present invention is based consists essentially in being able to switch off, during a gear change, the gear-change actuator of the previous gear, while also taking account of gear-changing comfort and the service life of the gear-changing apparatus involved. Gear-changing comfort can be impaired by the gear-changing jolt which may occur, and the service life can be shortened by excessively long sliding times of the rubbing surfaces.
The foregoing object has been achieved in an advantageous manner in accordance with the present invention, by providing a method and arrangement in which the reference parameter is varied as a function of a drive engine load torque and, to the extent necessary (i.e. the appropriate extent), a conversion ratio of a hydrodynamic torque converter is utilized, and the significant pressure value is associated with an instantaneous value of transmittable torque of the frictional connection of the new gear corresponding to an instantaneous actual value of the load torque.
In the arrangement and method for the automatic gear-changing of the change-speed gearbox in accordance with the present invention, a reference signal, which changes in a corresponding manner, is formed by processing operating parameters of the drive engine and, if necessary, of a hydrodynamic torque converter fitted downstream in the force path. On the basis of the load torque present, this reference signal determines the torque which has to be instantaneously transmitted by the frictional connection of the new gear. When the working pressure of the associated gear-change actuator reaches or exceeds the pressure value corresponding to this torque which has to be transmitted, the working pressure of the gear-change actuator of the previous gear is switched off.
The present invention also includes advantageous circuit and control technology, as well as gear-changing of the change-speed gearbox in which a first frictional connection is disengaged and a second frictional connection is engaged.
For a counter-rotating gear-change of the change-speed gearbox in which the second frictional connection is disengaged and the first frictional connection is engaged, the present invention also includes control measures and apparatus for switching off the working pressure at the first frictional connection as is the case for the switching off of the working pressure at the second frictional connection.